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Series of physical events in an exothermic reaction

  1. Jul 17, 2009 #1
    I am having trouble clearly/completely understanding the main cause of a transfer of thermal energy into the environment as a result of a bond formation.

    In terms of a very simple, generic, exothermic reaction...

    What I know...I can picture the atoms coming together through electrical attraction(an example of motion/thermal energy of the reactants/products), but what happens after the collision of atoms. How does the kinetic energy of the reactants/products actually cause kinetic energy of the atoms/molecules of the environment.

    Or maybe the transfer of thermal energy happens before the bond occurs.

    Please help me. I have been searching, but the results have not been fruitful for a layperson like me.

    Please use the quality of my description above as an indicator of my level of understanding. I don't believe that high level mathematics in this are necessary for my immediate interests.
     
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  3. Jul 17, 2009 #2

    Mapes

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    The kinetic energy of the reactants/products induces lattice vibrations (i.e., thermal energy) in the surrounding atoms via electrostatic attraction. As a physical analogy, picture an array of balls attached by springs, where some of the balls are magnetically attracted to each other. If two balls snap together due to magnetic attraction, their movement will induce vibrations in the array. Does this help?
     
  4. Jul 17, 2009 #3
    Yes, it does make sense. Thank you for the mental model.

    What then would explain the explosive nature of very violent reactions. I.E. The sudden large apparent expansions (perhaps I am misled and there are no real expansions at all?), where it seems that many molecules are propelled outward?
     
  5. Jul 17, 2009 #4

    Mapes

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    Can you give an example?

    My analogy was intended to describe the solid state, but if a phase transition is involved (e.g., if a lot of very hot gas is created), it doesn't apply; gas atoms don't behave as if they're connected by springs. However, they do move very fast, creating pressure waves that can push solid objects (e.g., remaining reactants or solid products).
     
  6. Jul 17, 2009 #5
    Example: Pure sodium metal in water.

    And with regards to your comment about gases not behaving in a manner consistent with the spring model; Does a liquid, to an extent, behave similarly to your solid state model?
     
  7. Jul 17, 2009 #6

    Mapes

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    Definite phase transition as the water boils. This can be enough to propel the sodium out of the water.

    I haven't studied liquids to any extent, so this will be a superficial answer: there is nearly as much bonding as in the solid state (we know this from comparing the heat of fusion to the heat of vaporization), but the bonds are broken much more frequently.
     
  8. Jul 18, 2009 #7
    I have been doing some thought experiments, so...

    Let me see if i can picture this correctly... In an explosive exothermic reaction:

    1. To form the bonds of the explosive material, perhaps black powder (but it could be something different), thermal energy caused some atoms to come together.

    2. They normally would not want to be together, but with enough force they can be pushed together.

    3. Depending on the physical properties of the atoms, you can get them to "hook" together. There is no real hook, just electromagnetism, maybe an electron or two were exchanged.

    4. The energy is stored now as chemical potential, the atoms don't want to be together, but they are hooked.

    5. In the presence of enough energy, the hooks can be broken, releasing the atoms/molecules that normally repel each other out in all directions.

    Ok. Critique this. Be honest but not too complicated. I would like the basic idea first.

    I hope this is not one of those cases where no one actually knows the basic idea!
     
  9. Jul 19, 2009 #8

    Mapes

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    It might be helpful to think not of hooks, but of energy wells (energy minima). Every compound consists of a collection of atoms in a stable or metastable state; that is, their configuration lies at a global or local energy minimum. Reactions take atoms/compounds from one minimum to another, just as a book might move from a stable position on one shelf to a new position on another shelf.

    Explosives are generally in a local energy minimum, like all compounds. With sufficient heat or shock, they can move to a global energy minimum, or at least a deeper local minimum. The process releases the difference in energy between the two configurations; in the case of explosives, the energy is released relatively quickly.
     
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